A multi fuel engine refers generically to any type of engine, boiler, heater or other fuel-burning device which is designed to burn multiple types of fuels in its operation. Multi fuel engines have application in diverse areas to meet particular operational needs in the operating environment. For example, a common use of multi fuel engines is in military vehicles so that vehicles in various deployment locations may run a wide range of alternative fuels such as gasoline, diesel or aviation fuel. In combat settings, for example, enemy action or unit isolation may limit the available fuel supply and personnel may need to resort the type of fuel available for usage from enemy and civilian sources. Multi fuel engines are also desirable where cheaper fuel sources, such as natural gas, are available, but an alternative or secondary fuel, such as diesel fuel, is needed for performance reasons (e.g., faster reaction to short term load demand), as a backup in the event of an interruption in the supply of the primary fuel source, or for other operational or engine performance conditions.
A multi fuel engine typically operates with a specified mixture of the available fuels. Where a liquid-only fuel mixture is specified, a liquid fuel, such as diesel fuel, gasoline or other liquid hydrocarbon fuel, is injected directly into an engine cylinder or a pre-combustion chamber as the sole source of energy during combustion. When a liquid and gaseous fuel mixture is specified, a gaseous fuel, such as natural gas, methane, hexane, pentane or any other appropriate gaseous hydrocarbon fuel, may be mixed with air in an intake port of a cylinder and a small amount or pilot amount of liquid fuel, such as diesel fuel, is injected into the cylinder or the pre-combustion chamber in an amount according to a specified substitution ratio in order to ignite the mixture of air and gaseous fuel.
In one exemplary multi fuel engine, when the gaseous fuel is required for the combustion cycle, liquid natural gas (LNG) is pumped by a cryogenic pump from a LNG tank through a vaporizer and into an accumulator as compressed natural gas (CNG). The accumulator typically is minimally sized to take the volume of one full stroke of the cryogenic pump. The cryogenic pump is powered by pressurized oil provided by a hydraulic pump that may be powered by an operative connection to an output shaft of the multi fuel engine or by a battery pack that is selectively operatively coupled to the output shaft for recharging. When connected to the engine, the mechanical hydraulic pump or the battery pack draw power from the engine that otherwise would propel the machine, and the hydraulic pump and the cryogenic pump must work continuously to provide CNG to the engine. The power draw for the hydraulic pump requires more power to be generated by the engine and, correspondingly, the use of more fuel by the engine, to propel the machine at the commanded speed. Where the hydraulic pump is electric and powered by the battery pack, the hydraulic pump may run off the battery pack for a time, but eventually the batteries must be recharged by the engine, and possibly while the engine is propelling the machine, and thereby resulting in similar inefficient operation of the machine. In view of this, a need exists for improved systems and controls for operating the hydraulic and/or cryogenic pumps to provide gaseous fuel to the multi fuel engine and, where natural gas is used as the gaseous fuel, converting LNG to CNG and providing the CNG to the engine for the combustion cycle in a manner that improves the efficiency of the machine system.